1. Field of the Invention
The present invention relates to data storage for computer sysplexes (“SYStem comPLEXES”), and more specifically, to monitoring serial and multiple concurrent access to data on a direct access storage device (DASD) shared among multiple sysplexes.
2. Description of the Related Art
A computer sysplex (“SYStem compPLEX”) refers to a tightly-coupled cluster of independent instances of an operating system (OS) such as z/OS®. A sysplex enables the linking of multiple system images within multiple machines and treating them all as a single image. A sysplex can be a monoplex, a basic sysplex or a parallel sysplex for example. What makes a group of systems or OS images into a sysplex is the inter-image communication.
A monoplex is a single stand-alone system that is configured to use sysplex services within the system. In a monoplex, applications such as DB2, CICS, IMS etc. can communicate to each other within the system.
A basic sysplex includes multiple computer systems that communicate using channel-to-channel (CTC) connections between logical partitions (LPARs). An LPAR is in practice equivalent to a mainframe running its own OS.
A parallel sysplex includes multiple computer systems or LPARs that communicate using a Coupling Facility (CF) or cross-system coupling facility (XCF). The CF or XCF may reside on a dedicated stand-alone server configured with processors that can run Coupling Facility control code (CFCC), as integral processors on the systems themselves configured as ICFs (Internal Coupling Facilities), or less common, as normal LPARs. The CF contains Lock, List, and Cache structures to help with serialization, message passing, and buffer consistency between multiple LPARs. The primary goal of a parallel sysplex is to provide data sharing capabilities, allowing multiple databases for direct reads and writes to shared data.
A sysplex may store data on a Direct Access Storage Device or DASD, which is any secondary storage device in which each physical record has a discrete location and a unique address. DASD may include, for example, disk drives, magnetic drums or data cells. DASD may use sequential, indexed or direct access methods. The direct access capability allows records on DASD to be accessed without having to read through intervening records from the current location. Direct access is much faster than sequential access.
A plurality of logical volumes is defined on DASD. Each logical volume comprises a volume table of contents (VTOC) that is a data structure that locates data sets on the logical volume and a virtual storage access method (VSAM) volume data structure (VVDS). The VVDS includes a VSAM volume control record (VVCR), a VVDS self-describing volume record and a plurality of virtual volume records (VVRs) for different data sets. Each VVR has fields that contain information related to a particular data set on the logical volume.
DASD is designed to be used within a single sysplex. The OS on each system includes access logic to “open” and “close” a data set. The internal communications within the sysplex e.g. the CF in a parallel sysplex enforces serialization or permitted multiple access to data sets on DASD.
A unit control block (UCB) is stored in memory for each system in the sysplex. The UCB is a memory structure or “control block” that describes any single peripheral device e.g. DASD. The UCB will include a field that indicates if DASD may be shared with other systems. For a base or parallel sysplex, this field must be set to allow shared DASD.